Conventional finishing methods for exterior doors and door system components, where a traditional finely finished wood grain appearance is desired, have involved staining a bare wood surface with dye and/or pigment-bearing stains followed by application of one or more coats of exterior varnish. A seal coat is sometimes applied prior to the stain and topcoat to seal the pores of the wood and to promote a more adherent and uniform stain and topcoat. The stain is virtually always solvent borne, as water based stains have the effect of raising the grain, thus requiring an intermediate sanding step which is labor and time intensive. In addition to the solvent contained in the stain, stains contain drying oils as carriers, for example heat-bodied linseed oil. Generally, several different oils are provided to promote penetration of the stain into the wood surface.
After the stain has dried, one or more transparent topcoats are applied. Traditional topcoats are varnishes containing dissolved resins such as copal, often in conjunction with a drying oil. More recently, chemically modified natural resins and synthetic polymers have replaced all or part of natural resins, especially for outdoor use. In addition, to provide superior weatherability, ultraviolet absorbers have been added. Despite the advances made in such finishes, varnishes still generally require light sanding or abrading with steel wool or the like between coats, or necessitate the addition of the second coat prior to the full drying of the previous coat. Otherwise, delamination between varnish coats may occur. Sanding and similar treatments are time consuming, and judging the proper time to apply a second coat onto a not fully-cured prior coat requires some skill and experience. Despite these drawbacks, stain/varnish systems continue to be used on exterior door systems.
Recently, high quality exterior door systems employing molded thermoset and/or thermoplastic composite skins have become commercially available. These door systems are compression molded to exhibit a wood grain appearance, and when properly finished, are difficult to distinguish from natural wood doors. These composite doors offer several advantages over natural wood products, such as freedom from warpage, lower cost, greater insulation efficiency, and the like. Unfortunately, the stain/varnish systems in use for finishing natural wood doors are only marginally acceptable for composite doors. A satisfactory finish may often be achieved under production conditions where skilled workers, sophisticated spray equipment, drying ovens and the like are available. However, such systems are ill-suited for application by the consumer.
The difference between the ease with which finishes may be applied to natural wood products on the one hand and composite exterior door systems on the other, is due to the nature of the materials involved. Natural wood is a porous and absorbent product. Hence, dyes may penetrate the wood fibers as well as into the wood pores. The latter may also serve to trap and retain pigment, as may partially abraded fibers generated in sanding the wood prior to finishing. However, composite door skins are constructed of sheet molding compound (SMC) or other thermoset or thermoplastic matrices, often containing fillers which may range from wood flour to finely ground minerals and/or glass fibers. Despite the presence of fillers, the composite door skins are essentially non-porous. Thus, stains containing dyes as the colorant are largely ineffectual, and pigmented stains require a higher amount of pigment than would be required for a conventional wood stain. The lack of porosity and the generally smooth surface of such products require the stain pigments to be physically adhered to the surface.
Furthermore, the drying oils used in conventional stains, particularly those of lower viscosity, act as subsurface emollients in wood, but remain as an exterior thin film on composite skins. Lower molecular weight oils, being unable to penetrate the essentially non-porous surface, may remain tacky or require lengthy drying times or oven cure. During weathering due to light exposure, these oils experience scission of naturally occurring unsaturated molecular sites. The scission produces a variety of lower molecular weight, liquid and often volatile components, which causes the subsequently applied topcoat to blister, coming off in sheets.
The detrimental effects of drying oil vehicles may be minimized by substituting quickly evaporating solvents such as mineral spirits, naphtha, or the like. However, such stains tend to dry rapidly, leaving application marks, i.e., "brush marks" where overlapping coats of stain are applied. Elimination of drying oil components results in little adherence of pigment to the surface. This minimal adherence may be acceptable in porous products, but is highly problematic when non-porous surfaces are involved.
Adkins, et al., in U.S. Pat. No. 4,923,760, discloses water-borne emulsions employing two different maleinized linseed oils together with cosolvents such as propylene gylcol tertiary butyl ether and a variety of hydrocarbons. However, such stains are still formulated for porous wood surfaces and are not well suited to non-porous composite door skins. Lowering the amount of the oil vehicles would be appropriate, but the stains then dry too fast and have only a limited capacity to stably disperse pigment.
Pigmented stains, whether solvent-borne or water borne, are traditionally protected by aromatic polyurethane topcoats. When the previously deposited stain contains low levels of scissionable drying oils, failure by blistering and peeling is substantially minimized. However, aromatic polyurethanes are subject to severe degradation by light. Exposure to sunlight causes degradation which ultimately results in ablative loss of the coating when applied to non-porous surfaces. Polyurethane varnishes often contain sacrificial ultraviolet (UV) protectants. However, the ultraviolet light protection package is typically consumed in as few as eight months exposure to sunlight. As the stain coat is not strongly adhered to non-porous substrates, failure of the topcoat results in ablative loss of the stain layer, typically in less than two months after the topcoat is breached.
Aqueous topcoats have been used with some success, as indicated by the disclosures of U.S. patents to Gobel et al. (U.S. Pat. No. 5,342,882) and Sanders et al. (U.S. Pat. No. 4,509,981). Water-based topcoats employing aliphatic urethane-acrylic latexes are commercially available. The use of a non-sacrificial UV protectant package in these coating compositions provides a substantial increase in durability of underlying resin-bound solid pigmented coating relative to the degree of protection provided by aromatic polyurethane finishes. However, these finishes are incompatible with pigmented stains having little or no resin binder and high pigment loading. This incompatibility is believed due to the relative differences between the somewhat hydrophilic pigment particles and the hydrophobic topcoat.
It would be desirable to provide a pigmented stain suitable for use on relatively non-porous thermoset and thermoplastic composite surfaces, particularly door skins. It would be further desirable to provide such pigmented stains in a formulation which can easily be applied by the consumer in a uniform manner, without resort to the use of expensive and not readily available production equipment. It would be yet further desirable to provide a topcoat which is compatible with the pigmented stain, and which exhibits a high, continuing degree of ultraviolet protection and weathering resistance. The stain/topcoat system should not employ two-component coatings with limited pot life, and should be removable by traditional paint strippers should the need arise.